The ability to sense and respond to the changing light environment is a fundamental feature of living organisms. The blue/UV-A light receptor cryptochromes is the only photoreceptor that functions in both plants and animals to regulate photoperiodic responses and the circadian clock, which have been associated with not only plant development, but also various human health problems including sleeping disorders and behavioral disorders. We have recently discovered that Arabidopsis cry2 is a major photoreceptor controlling photoperiodic flowering in plants. We have also found that a calcium-binding protein SUB1 is one of the downstream signaling components of cry2. We hypothesize that protein-protein interaction is associated with the early signaling process of cry2 and that regulation of gene expression is a major mechanism underlying cry2 function. We propose to continue investigate the signal transduction mechanism of Arabidopsis cry2. The objectives of this proposal are: (1) to identify and characterize the genetic suppressors of the sub1 mutant, and to clone and characterize the SUB4 gene and test a hypothesis that SUB4 may act as a negative regulator of cry2. (2) to investigate how cry2 regulates photoperiodic flowering by identifying genes for which the expression is regulated by cry2. (3) to continue characterize a cry2-interacting protein that we have recently isolated, and to isolate and characterize additional cry2-interactinig proteins. (4) to isolate and characterize Arabidopsis mutants impaired in blue light-induced cry2 degradation and blue light-dependent induction of transcription.